One function of territorial defense may be to facilitate mate guarding by resident males. To assess the importance of mate guarding in territoriality, we examined the spatial pattern of intrusions by male Willow Ptarmigan (Lagopus lagopus) on the territories of other males and we compared use of territorial space by paired and bachelor males. Because intruding males tended to remain near territorial boundaries of other males more than expected by chance, we expected paired males to avoid areas near boundaries, where the chances of the female encountering another male would be higher. We found that before females had settled onto territories, all resident males used boundary areas of their territories extensively. After females settled, however, paired males remained almost exclusively in the cores of their territories, whereas bachelor males continued to use primarily the edges of their territories. Those patterns of space use suggest that the benefits of having more exclusive access to a mate may be one of the selective forces driving territorial behavior in this species.